1. Field of the Invention
The present invention relates to environmental analysis and, more particularly, to an improved method and apparatus for spiking air sample "sorbent tubes" with reference compounds.
2. Discussion of Related Art
To analyze gas samples using, for example, a gas chromatograph or mass spectrometer, the gas to be analyzed is collected in the field. When the sample is to be tested, it is collected on tubes known as "sorbent lubes". FIG. 1 is a cross-sectional view of a conventional sorbent tube 50. The sorbent tube 50 includes a cylinder 52 having openings 54, 56 at both ends. Inside the cylinder 52 is sorbent material 58. This sorbent material may be comprised of various materials, such as activated charcoal (either tenex, a carbon based material, or otherwise). The sorbent material is used either individually or in combination with other materials. When used in combinations, the sorbent material 58 is arranged in layers (layer boundaries 59 are shown in FIG. 1) so the weakest sorbent is at the inlet opening 54, followed by progressively stronger sorbents. (In FIG. 1, the sorbent material strength is represented by the number of dots per unit area. The greater number of dots/area represents greater sorbent strength.) This allows heavier molecules (e.g., larger molecules) in the sampled gas to cling to the weaker sorbent material and lighter molecules (e.g., smaller molecules) to cling to stronger sorbent material. This traps the molecules according to the size and/or polarity of the molecules. The sorbent tube openings 54, 56 are capped with gas tight seals or fittings 60, 62.
As seen in FIG. 2, when the sample is to be analyzed, the sorbent tubes 50 are loaded onto a tube desorber 64 connected to a sample concentrator 66. The concentrator 66 is connected to an analysis device 68, such as a gas chromatograph and/or mass spectrometer device 68. The sample is removed, or "desorbed", from the sorbent tube 50 to the concentrator 66 using a purge gas such as helium or nitrogen. The sample is loaded into the sorbent tube with the weaker sorbent material closer to the sample than the stronger sorbent material. Thus, the heavier molecules cling to the weaker sorbent material and the lighter molecules pass through the weaker sorbent material and cling to the stronger sorbent material. The sample is then removed, or "desorbed", from the sorbent tubes. This is done by heating the sorbent tube 50 and applying a gas flowing, the direction opposite of that used to collect the sample. This allows the molecules to dislodge from the sorbent material 58.
Known amounts of reference substances, such as reference gases, are typically added to each sorbent tube 50 after the sample is loaded into the tube, but before the sample is desorbed. This is called "spiking" the sample. Samples are spiked for several reasons. One reason is to tune the analysis device 68; a second reason is to calibrate responses of the device 68; and a third reason is to provide internal standards and surrogate compounds. Each is briefly explained.
Regarding the first reason, a mass spectrometer may need to be "tuned" to determine if it is operating properly. The device may be tuned by analyzing one or more reference substances each having a known mass spectrum. The analysis results may be compared with a known mass spectrum to determine if the device is properly tuned.
Regarding the second and third reasons, samples of some or all of the compounds expected to be found may be analyzed to determine the characteristics of each. This permits determination of responses of compounds to be analyzed relative to internal standard compounds. During the calibration process, known amounts of compounds of interest and internal standards are analyzed. Responses of compounds and internal standards are compared. Responses for compounds of interest relative to the internal standards are determined and are referred to as response factors. For example, during a calibration, a known amount of a compound of interest (methylene chloride, for example) and a known amount of internal standard (bromochloromethene, for example) are analyzed. If the relative response of the compound of interest is one half that of the internal standard, this response may be used to determine the quantity of the compound of interest in the sample to be analyzed.
In the sample to be analyzed, concentration of the compounds of interest are unknown. Known amounts of internal standards, however, have been added to the sample prior to the analysis. The concentration of compounds of interest may be calculated using the response factors determined in the calibration step. For example, a sample is spiked with 100 ngs of bromochloromethene, and methylene chloride is detected. The amount of methylene chloride in the sample may be quantified with reference to the relative response of methylene chloride that had been determined in the calibration step. That is, if the detected methylene chloride had an analytical response equal to 100 ngs of bromochloromethene, and its relative response was one half that of bromochloromethene, the quantity of methylene chloride may be determined to be 200 ngs.
Also regarding the third reason, because known amounts of the reference substance are added to the sample, the efficiency of the desorbing process may be measured by the amount of the reference substance detected by the analytical system. This is called a surrogate compound. If, for example, only 20% was recovered of a first reference substance having a certain molecule weight, and 75% was recovered of a second reference substance having a different, higher molecular weight, it is possible that the detected gases near the molecular weight of the first reference substance may be 80% higher than measured, and the detected gases near the molecular weight of the second reference substance may be 25% higher. Alternatively, the results may show that the desorbing process was not properly performed, and the results should be questioned or suspect.
Previously, several methods have been used to spike samples. Two well known methods are: (1) using a liquid solution and (2) using a gas solution. Each is described below.
Liquid solutions include small amounts of reference substance in solution with a solvent, such as methanol. The amount of the reference substance in the solution is typically on the order of nanograms. The concentration of the solvent may be a million times higher than the compounds of interest. Reference solutions are typically added to sorbent tubes by a process "flash evaporation". This process involves heating an injection port with inert gas flow. This vaporizes the solution of compounds of interest and the solvent. This mixture is then carried onto the sorbent tube.
After the sample has been loaded onto the sorbent tube 50, a syringe is used to manually inject the reference solution onto the solution into the sorbent tube 50 through one of the seals/fittings 60, 62. This method has several drawbacks. Solvent loading of the sorbent tube can lead to various problems. For example, the detector/analyzer response may be diminished by the presence of high concentrations of any individual component of the sample. The high solvent concentrations (as much as a million times higher than the reference substance) can cause chromatography problems and may reduce the response co-eluting compounds.
Spiking a sorbent tube with a gas solution is performed as follows. First, a gas solution must be prepared. Although it is possible to purchase (or prepare) pre-mixed, high pressure canisters of gas spiking solutions, it is often a cumbersome process. Typically, a gas solution is manually mixed for each tube 50 to be spiked. These solutions are difficult to prepare because known volumes of each components of the mixture must be manually added to the solution. There may be as many as sixty or more compounds in a gas solution mix. Each of these compounds must be obtained in pure form. This process may be very expensive and time consuming. Gas mixtures go bad quickly. Some reasons for this are leaks, condensation of components, and concentration changes that occur with use, particularly when stored at or near atmospheric pressure. Another problem with gas mixtures is that it is difficult to measure known mounts of the mix. Gas volumes are usually measured with syringes or mass flow meters, both of which permit the introduction of human error.
Both methods have other drawbacks in common. Both procedures are relatively-time consuming. Typically, it may take several minutes to spike a single sorbent tube using the liquid solution method and many minutes using the gas solution method. Most of this time involves preparing the solution. Both methods adversely affect the reproducibility of the procedure because of the inaccuracies involving the technique (both human and hardware errors). The inaccuracy of these methods reduce the reliability of he analysis results. Another drawback caused by both liquid and gas solutions is that manual injection introduces opportunities or human and/or hardware errors, making it difficult or impossible to precisely control the mount of solution injected into the sorbent tube. As described above, however, precision is desired because the analysis often depends on the mount of the reference substance present in the sorbent tube, particularly when spiking for internal standards and surrogate compounds.
Therefore, it is an object of the present invention to provide a method and apparatus for spiking a sorbent tube with a precisely measured mount of reference substance.
It is another object of the present invention to provide a method and apparatus for spiking a sorbent tube in a highly reproducible manner.
It is yet another object of the present invention to provide a method and apparatus for quickly spiking a sorbent tube with a substance.
It is yet even another object of the present invention to provide a method and apparatus having a high percentage of accuracy.